The present invention relates in general to the cooling of nuclear reactors and in particular to a new and useful mixer grid for cooling the fuel rods of a nuclear reactor fuel assembly.
In the nuclear power field, it is common to use a mixer grid in conjunction with a nuclear reactor fuel assembly for enhancing the heat transfer between the nuclear fuel rods of the assembly and the coolant of the nuclear reactor.
U.S. Pat. No. 4,474,730 discloses a nuclear fuel spacer grid comprising a plurality of straps in an egg crate configuration defining a plurality of cells for enclosing the fuel rods for a nuclear reactor fuel assembly. A spring portion is provided on at least one of the straps of each cell and extends into each cell for pushing against the fuel rod contained therein. A back-up spring is positioned transverse to and behind each of the spring portions of the straps and is positioned so as not to touch the fuel rods within the cells.
U.S. Pat. No. 3,933,584 discloses a spacer grid for nuclear fuel rods comprising identical metal straps arranged in cross relation in order to define a multiplicity of cells for receiving elongated fuel elements such as rods. Tabs are used at the edges of the cell openings for the deflecting of coolant into the cells. Tabs from adjacent side walls are fixedly secured together in order to provide rigid flanges for the grid. Spring fingers are provided within each cell in order to hold the fuel rods against fixed stops.
U.S. Pat. No. 4,692,302 discloses a mixer grid for a nuclear reactor fuel assembly wherein the sole purpose of the grid is to promote cross flow mixing of the coolant of the reactor rather than support the fuel rods. At least one mixing vane is provided in each cell for directing coolant flow. Four dimple protrusions, which are open, extend into each cell and allow the flow of coolant therethrough and prevent damage to the fuel rods within each cell.
A major problem of the mixer grids commonly used in the nuclear reactor field is that both a separate stopper and mixing vane are used within the same cell for cooling hot spots within the grid and supporting the nuclear fuel rods. Hard stops used within each cell are aimed at preventing the fuel rods from contacting the mixing vanes of each cell while the mixing vanes are used to direct and channel coolant in each cell.
Additionally, it is common in the known mixer grids to use an outer strip around the grid for providing lead-in and coolant flow to hot spots of cells located on the periphery of the grid.
Moreover, it is common in the known mixer grids to use mixing vanes in every other peripheral cell of the mixer grid.
Presently, there is no known mixer grid which alleviates the need for using both a stopper and a mixing vane within each cell of the grid.